“…The dimethylzirconium complexes were observed to decompose under prolonged dynamic vacuum, so care was taken to avoid extended exposure of these materials to vacuum in the solid state. Similar observations were made with (Cp) 2 TiMe 2 . − …”
A new class of group 4 complexes bearing the tropidinyl (Trop) ligand were synthesized. This novel anionic ligand, easily derived from the natural product tropine, is regarded as an analogue of the well-studied Cp ligand (Cp ) cyclopentadienyl) and features a bicyclic ring system in which an allyl function donates 4π electrons and an amine function donates 2σ electrons to the metal center. The synthesis of the group 4 complexes involved transmetalation of a stannylated allyl functionality with a group 4 metal halide with concomitant elimination of Me 3 SnCl. Reaction of endo-stannylated tropidine (1) and ZrCl 4 , CpZrCl 3 , or CpTiCl 3 afforded the complexes (Trop) 2 ZrCl 2 (2), (Cp)(Trop)ZrCl 2 (3), and (Cp)-(Trop)TiCl 2 (4), respectively. These dihalide complexes were alkylated using methyllithium to generate (Trop) 2 ZrMe 2 (5), (Cp)(Trop)ZrMe 2 ( 6), (Cp)(Trop)TiMe 2 (7), or benzylmagnesium chloride to give (Trop) 2 ZrBn 2 ( 8) and (Cp)(Trop)ZrBn 2 (9). Complexes 2, 3, 5, and 8 were characterized by X-ray crystallography. Complexes 5 and 6 were observed to undergo protonolysis with various acids (HA) to give mixed [Zr]MeA complexes. Reaction of complex 6 with isonitriles (ArNC) gave the η 2 -iminoacyl complex (Cp)(Trop)ZrMe[N(Ar)CMe] (13), whose structure was confirmed by X-ray crystallography. All the complexes displayed dynamic behavior, and the various exchange processes were investigated by variabletemperature NMR spectroscopy. The dihalide complexes 2, 3, and 4 in the presence of modified methylaluminoxane (MMAO) were active catalysts for the polymerization of ethylene. The catalyst system 3/MMAO displayed a very high ethylene polymerization activity and was similar to that observed for Cp 2 ZrCl 2 /MMAO. High molecular weight polypropylene was generated using 4/MMAO, and polypropylene oligomer was obtained using 3/MMAO. The cationic complex [(Cp)(Trop)ZrMe][MeB(C 6 F 5 ) 3 ] (14) was formed upon reaction of the dimethyl complex 6 with B(C 6 F 5 ) 3 and was shown to be an active ethylene polymerization catalyst.
“…The dimethylzirconium complexes were observed to decompose under prolonged dynamic vacuum, so care was taken to avoid extended exposure of these materials to vacuum in the solid state. Similar observations were made with (Cp) 2 TiMe 2 . − …”
A new class of group 4 complexes bearing the tropidinyl (Trop) ligand were synthesized. This novel anionic ligand, easily derived from the natural product tropine, is regarded as an analogue of the well-studied Cp ligand (Cp ) cyclopentadienyl) and features a bicyclic ring system in which an allyl function donates 4π electrons and an amine function donates 2σ electrons to the metal center. The synthesis of the group 4 complexes involved transmetalation of a stannylated allyl functionality with a group 4 metal halide with concomitant elimination of Me 3 SnCl. Reaction of endo-stannylated tropidine (1) and ZrCl 4 , CpZrCl 3 , or CpTiCl 3 afforded the complexes (Trop) 2 ZrCl 2 (2), (Cp)(Trop)ZrCl 2 (3), and (Cp)-(Trop)TiCl 2 (4), respectively. These dihalide complexes were alkylated using methyllithium to generate (Trop) 2 ZrMe 2 (5), (Cp)(Trop)ZrMe 2 ( 6), (Cp)(Trop)TiMe 2 (7), or benzylmagnesium chloride to give (Trop) 2 ZrBn 2 ( 8) and (Cp)(Trop)ZrBn 2 (9). Complexes 2, 3, 5, and 8 were characterized by X-ray crystallography. Complexes 5 and 6 were observed to undergo protonolysis with various acids (HA) to give mixed [Zr]MeA complexes. Reaction of complex 6 with isonitriles (ArNC) gave the η 2 -iminoacyl complex (Cp)(Trop)ZrMe[N(Ar)CMe] (13), whose structure was confirmed by X-ray crystallography. All the complexes displayed dynamic behavior, and the various exchange processes were investigated by variabletemperature NMR spectroscopy. The dihalide complexes 2, 3, and 4 in the presence of modified methylaluminoxane (MMAO) were active catalysts for the polymerization of ethylene. The catalyst system 3/MMAO displayed a very high ethylene polymerization activity and was similar to that observed for Cp 2 ZrCl 2 /MMAO. High molecular weight polypropylene was generated using 4/MMAO, and polypropylene oligomer was obtained using 3/MMAO. The cationic complex [(Cp)(Trop)ZrMe][MeB(C 6 F 5 ) 3 ] (14) was formed upon reaction of the dimethyl complex 6 with B(C 6 F 5 ) 3 and was shown to be an active ethylene polymerization catalyst.
“…The syntheses of 1 − 5 were typically conducted in the absence of light, but where they have been repeated without this precaution, there were no obvious detrimental effects. This is in contrast to the behavior of the isolobal titanocene systems …”
A series of alkyl- and aryl-imido titanium dialkyl compounds Ti(NtBu)(Me3[9]aneN3)R2 (R = Me (1),
CH2SiMe3 (3), CH2
tBu (4), CH2Ph (5)), Ti(NR)(Me3[9]aneN3)Me2 (R = iPr (6), Ph (7), 3,5-C6H3(CF3)2
(8), 2,6-C6H3
iPr2 (9), 2-C6H4CF3 (10), 2-C6H4
tBu (11)), and Ti(NR)(Me3[9]aneN3)(CH2SiMe3)2 (R = iPr
(12), ArF (13)) were prepared and crystallographically characterized in the case of 1, 6−9, and 11 (Me3[9]aneN3 = 1,4,7-trimethyl triazacyclononane; ArF = C6F5). These compounds, isolobal with the titanocenes
Cp2TiR2, were thermally stable at elevated temperatures except for 4. Reaction of 7 with [Ph3C][BArF
4]
(TB) and diisopropylcarbodiimide in CH2Cl2 gave the Ti−Me insertion product [Ti(NPh)(Me3[9]aneN3){MeC(NiPr)2}][BArF
4] (15-BAr
F
4
). The corresponding reaction of 7 in the absence of organic
substrate gave [Ti2(μ-NPh)2(Me3[9]aneN3)2Cl2][BArF
4]2 via a solvent activation reaction. The room-temperature ethylene polymerization capabilities of the dialkyl compounds were evaluated using TB
cocatalyst in the presence of AliBu3 (TIBA). Among the dimethyl precatalysts, only the systems 1 and
11, with the bulkiest imido groups, showed high productivities (6230 and 1210 kg mol-1 h-1 bar-1,
respectively). The productivites of the other tert-butyl imido precatalysts 3 and 4 (130 and 120 kg mol-1
h-1 bar-1, respectively) were substantially lower than that of 1. The catalyst system 1/TIBA (2500 equiv,
no added TB) was also active for ethylene polymerization (225 kg mol-1 h-1 bar-1). The less productive
imido dialkyl precatalysts all formed complex mixtures on exposure to TIBA. The polyethylenes produced
with 1, 3, and 5−11 generally had M
w/M
n
values in the range 2.6−3.0. The PE formed with 1/TB/TIBA
was terminated only by methyl end groups, consistent with chain transfer to TIBA followed by subsequent
β-H transfer by the resultant titanium isobutyl cation. The alkyl cations [Ti(NtBu)(Me3[9]aneN3)R]+ (R
= Me or CH2SiMe3) reacted rapidly with TIBA in C6D5Br at −30 °C, forming isobutene. DFT calculations
found that TIBA adducts of the model methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ were energetically
favorable by ca. −80 to −110 kJ mol-1. Whereas 1 alone or with AlMe3 present has been shown to form
only Ph3CMe on reaction with [Ph3C]+, 1:1 mixtures of 1 and TIBA gave Ph3CH as the only trityl-containing product, suggesting a key role for transient [AliBu2]+ in the activation process for these catalysts.
Overall, the imido group in the Ti(NR)(Me3[9]aneN3)Me2/TB/TIBA catalysts systems appears to have
two roles: to stabilize the dialkyl precatalyst toward degradation by the TIBA itself prior to activation,
and to inhibit the formation of catalytically inactive hetero- or homo-bimetallic complexes.
“…Crystalline PhTi(O i -Pr) 3 was the first isolated and fully characterized organotitanium compound. , Methyl- and other monoalkyltitanium derivatives are also reasonably stable, but di-, tri-, and tetraalkylated derivatives all appear to be considerably less stable. Quantitative data were obtained for the thermal decompositions of methyl-, − benzyl-, − and neopentyltitanium(IV) derivatives ,− to evidence that free radicals are not formed in these decomposition reactions, at least not in noncoordinating solvents. The relatively lower stability of the organotitanium compounds is definitely not connected with a particular weakness of the carbon−titanium σ-bonds (in fact, these bond energies usually exceed 60 kcal mol -1 ) but rather with the readiness of titanium to provide its low-lying empty d-orbitals for so-called agostic interactions with neighboring σ-bonds. ,− In the trichloro(methyl)titanium ethylenediphosphine chelate 1 , R = Me, the methyl group displays a pronounced distortion with a Ti−C−H angle of 70(2)°, , and this phenomenon has been explained with an agostic interaction between the titanium atom and the C α −H bond …”
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